Combined cycles power plants are known to include a gas turbine engine and a steam turbine engine. The steam turbine engine has a boiler that is supplied with the exhaust gas from the gas turbine engine to produce steam that is expanded in a steam turbine. The steam turbine engine and gas turbine engine drive one or more electric generators that are connected to an electric grid to supply electric power to it.
As known, the frequency of the electric power of the grid has a nominal value that is fixed (50 Hz or 60 Hz) and the deviation from this nominal value shall be kept to a minimum.
Nevertheless, because of mismatch between the electric power supplied to the grid by the power plants connected to it and the electric drained from the grid by consumers connected to it, the frequency of the grid can deviate from the nominal value.
When this happens, some power plants connected to the grid must be regulated in order to bring the frequency of the grid back to the nominal value.
Typically, the frequency of the grid may increase above or decrease below its nominal value.
In case the frequency drops to a value lower than the nominal value, the power plants connected to the grid must provide additional electric power to the grid in a very short time, to restore the grid frequency.
In case the frequency increases to a value higher than the nominal value, the power plants connected to the grid must reduce the electric power provided to the grid in a very short time, to restore the grid frequency.
Traditionally, this change in power supplied to the grid is achieved by increasing or reducing the power generated at the gas turbine engine by increasing or reducing the fuel provided in the combustion chamber of the gas turbine engine (this power is then converted through a generator in electric power that is supplied to the grid).
Nevertheless, when the combined cycle power plant is operated at a low operating point (i.e. at a low fraction of the nominal or full power of the gas turbine, for example 40%) a fast regulation of the gas turbine is not possible.
In fact, a regulation of the gas turbine at low load could cause unacceptable un-combusted hydro carbon and/or NOx emission, pulsations, flame extinction.